That's because bike manufacturers are putting batteries and motors into bikes, giving an electric boost that can shorten miles and flatten hills. Children have always marveled at the freedom bikes offer. Now, e-bikes can give adults that sense of liberation, too.

The feeling is so strong it convinced Don DiCostanzo, who lives at the top of a hill in Corona del Mar, California, to found e-bike company Pedego. "I liked to ride to the beach, but I hated to ride back," he says. E-bikes make the ride easy, and now he's selling 12,000 e-bikes a year in more than 60 stores worldwide.

DiCostanzo, 58, focuses on baby boomers looking for recreation or wanting to keep up with the grandkids. "Ninety-nine percent of our customers would never have purchased another bike in their lifetime if not for us," DiCostanzo says.

But don't think e-bikes are just for older folks. Serious riders are embracing e-bikes to commute to work. And they are opening the great outdoors to people who don't feel fit enough to bike up hills and mountains without a boost.

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Fun factor

Several things are pushing e-bikes mainstream. Modern batteries store more energy. City dwellers crave better transportation. Electric cars have made battery power fashionable while showing the technology to be environmentally friendly.

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Worldwide e-bike sales are expected to grow from 32 million in 2014 to at least 40 million in 2023, says Navigant Research. Most of that growth will be in Europe and Asia, where people often use bikes to commute.

"In the US, the bike is more sporting equipment than a means of transportation," says Claudia Wasko, a director at Bosch, a top motor and battery supplier. But that's likely to change, she says. "The US market will follow the example of Europe."

"It's a fantastic fusion -- one of those rare occurrences where two different technologies come together and create something far greater than either one on its own," says Bjorn Enga, founder of boutique e-bike maker Kranked near Vancouver, British Columbia.

E-bikes conquer any hill in San Francisco, adds Dave Rodriguez, former marketing chief of Dylan's Tours, which has 30 Pedego e-bikes. "It's darn near driving a moped in a bicycle lane. It's too cool for school."

But they also undermine the very definition of a bicycle. Can you really call it biking if you barely have to pedal?

Choices galore

E-bikes fall into two categories. Pedal-assist models amplify the power of your legs with a motor that kicks in when you push the pedals. And throttle models boost the power when you twist a handlebar-mounted actuator, similar to revving a motorcycle engine. Battery packs on a bike's frame or rack supply power to a motor placed near the pedals or built into the rear wheel's hub.

Jonesing for an e-bike? You could just swap in Superpedestrian's $950 Copenhagen Wheel to give your conventional bike an electric boost.

It's hard for authorities to categorize e-bikes, too. Check your local regulations because there are different limits on motor power and even whether you're allowed to ride in bike lanes or use throttle controls (instead of pedals) for power.

Ranges vary. Specialized estimates a single charge will power its pedal-assist Turbo S 35 miles on flat terrain. Kranked's throttle-operated mountain bikes, which cost between $5,200 and $10,000, are powerful enough to be ridden up steep trails and go 25 miles at full tilt.

Motorized cheats

Although cyclists love the idea of e-bikes replacing cars on the road, the thought of electric mountain bikes is more divisive. Are they bicycles or motorized vehicles?

"Most serious mountain bikers don't look at an e-mountain bike as anything they'd ever be interested in," says Mountain Flyer magazine editor Brian Riepe. Although motors open the sport to people who might not otherwise participate, "lots of the purist mountain bikers are going to turn their noses up at e-bikes."

E-bikes also could alienate other trail users and even undo painstaking victories securing mountain bike access. The risk: People could associate e-bikes with motorcycles that can permanently scar fragile soil and vegetation.

Famous Utah rides like the Slickrock Trail are off limits after the Bureau of Land Management's 2014 decision to restrict "motor-assisted bicycles" to roads that permit motorized vehicles. The International Mountain Bike Association, after years of fighting for trail access for bikes, agrees with that decision. "We're erring on the side of caution. If it has a motor, it has a motor," says communications director Mark Eller.

But Kranked's Enga says naysayers are "driven by fear of losing what they've got rather than embracing things that allow more people to get on the trails and enjoy it."

More than likely, the two sides will eventually have a meeting of the minds. Biking will accommodate motors as it has gears and suspension. "If you climb Mount Everest, does it count if you get to the top with a Sherpa pushing you the whole way?" asks Electra President Kevin Cox.

Cheating or not, people will do it.

This story appeared in the fall edition of CNET Magazine. For other magazine stories, click here.

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